Compressibility is only worth considering if your flow velocity exceeds a Mach number of 0.3. The incompressible flow solver is much more efficient and robust for flows below this threshold. If your case is likely to be compressible then review the settings for the tutorial "Transonic Flow Over the NACA 0012 Airfoil". Note that you need to precisely specify the reference temperature and pressure to match the conditions you are simulating.